Best way to crank engine for a compression Test?

[Tim]

Just want to make sure I don’t burn anything up on my 2002 RT 10 doing my compression test,
Do I remove the Auto Shut Down Relay from the PDC?
Then I can go ahead and crank with the key?
Things have changed since my old muscle cars and a little mistake today can be expensive and I don’t want to start using jumpers and a bump switch as I am not too sure where to plug what into any more.

Thanks

Tim

 

[Joseph Dell]

Pull the fuel-pump relay in the trunk. This way it won't dump fuel when you crank.

Ideally, spend $10 on a bump-starter (napa or any other auto parts store should have one). Just make sure the car is out of gear!!

Good luck!

 

[Tim]

I have the bump starter switch but there is no relay on the fender to hook it to. Do I pull the relay on the PDC (power distribution center) and then clip on the # 87 on the relay connector of the PDC and the other to the 12Volt post next to the PDC?

Thanks

Tim

 

[BOTTLEFED]

A simple way I have done it is the bump switch connected directly to the starter, doesn't matter how, but connect one clip to the bigger post from the battery and the other to the solenoid (smaller post on starter solenoid).

Then just disconnect the battery + post above the PCM or, if you have a VEC, remove the fuse from it. That will keep the fuel and ignition from activating. I usually remove the fuel pump fuse for extra piece of mind, but it won't run since you won't have the key in the ON position.

 

[grcforce327]

Just buy a bumper switch,attach to the posts on the starter.Leave the ignition off(no key).No need to touch or pull anything on the car!Hit the bumper switch and the motor turns.Good for adjusting valves etc!

 

[ViperTony]

Tim, I found the easiest way is to purchase a cheap bump start from AutoZone and attaching its two cable to the starter motor located just under the motor on the driver's side near the firewall. I recently did a compression test and snapped a couple of pics. The attached pic shows where the starter is located. I circled the mounting bolt to show the location. On the starter, there are two connection points to hook up the bump starter cables. Igniton off and keep it neutral before you bump start. It may be a good idea to keep a battery tender on the car if you're going to be playing with the bump starter for a long time.Hope this helps.

 

[Tim]

Electronics have changed since my old Chargers and Cudas and I just wanted to make sure that this part had not. The 2 posts on the starter and solenoid is the same, I will keep a battery tender on and with the key off there should be no need to pull anything else or disconnect anything else and I will keep it in neutral.

After changing heads, cam, intake, headers etc I am down a little on power (still over 525 hp to the rear wheels) but now I’m trying to find out if this is it or I still have some in the bank due to other problems.

I will keep everyone posted once it is all sorted out.

Thanks

Tim

 

[Tim]

I have 153 psi as my high with a 143 as the low with an average of 148 psi. So considering my altitude here of 2500 ft asl, the performance cam and the engine was warm by the time I finished this I would say that this is good. I had 8" of vacum at an idle and is all I will probally get and is probally normal.

Tim

 

[RedSrt007]

I have 153 psi as my high with a 143 as the low with an average of 148 psi. So considering my altitude here of 2500 ft asl, the performance cam and the engine was warm by the time I finished this I would say that this is good. I had 8" of vacum at an idle and is all I will probally get and is probally normal.

Tim

I get about 11-13" at idle depening on temperature (engine cold, fully warmed up, etc..)

 

[Tim]

Hey Red, are you stock with these numbers?

Thanks

Tim

 

[RedSrt007]

Hey Red, are you stock with these numbers?

Thanks

Tim

No, but when I was stock, I was @ 11"

Did you check for intake manifold leaks / vacuum leaks??

 

[Tim]

Yes I have checked all of the hoses, listened to it and I can’t hear any whistle and find any hoses off of it. I am at 2500 ASL with the Stryker Heads, Cam, Ported Intake etc, so maybe this is normal for my set up.

 

[ViperTony]

Tim, I was curious, you said you changed the intake. I assume you also went with larger TB's to make the porting on the intake. I wonder if the bigger TB's and Ported intake is hurting your engine's performance?

 

[Tim]

I did not change the TB. From what I have read the stocks ones work good and to go with the larger TB I would loose too much torque on the bottom end so I stayed stock. The intake is ported to match the heads as well I have the Bellenger Headers.

 

[Jack B]

Pull the fuel-pump relay in the trunk. This way it won't dump fuel when you crank.

Ideally, spend $10 on a bump-starter (napa or any other auto parts store should have one). Just make sure the car is out of gear!!

Good luck!

Unless I am missing something, with the key off and cranking off the starter, the injectors are not firing. There is no reason to crank with the key, the bump switch is too easy. I am also 11-13" at idle. With a Snap-On pressure gauge I show 160 psi cold, the compression ratio is about .5 psi higher than stock. It is a stock 97 cam, but, 1.7 lifters.

 

[wastntim]

525rwhp with heads, cam, and headers? I would have thought you should be closer to 575rwhp?

 

[ViperTony]

I did not change the TB. From what I have read the stocks ones work good and to go with the larger TB I would loose too much torque on the bottom end so I stayed stock. The intake is ported to match the heads as well I have the Bellenger Headers.

Ok. I ask this because I've been debating larger TB's and will probably mimick your upgrades at some point so I appreciate the info. Another question: I assume you have a tune to go along with the upgrades? SCT, Vec, PCM reflash? How's your A/F ratio?

 

[Tim]

Based on what I have read on here and have been told, 550 HP at the rear wheels would have been a minimum with an average of somewhere around the 575. I have a reflash from Dan and I’m waiting for another one to arrive for the SCT tuner.

A/F ratio is 12.6 to 4250 rpm then climbs to 13.4 to 5500 rpm where it climbs to 13.8 at 5700 rpm then drops to 13 at 6000 rpm.

My HP of 525 and torque of 524 are corrected back to sea level.

I will do a back pressure test on the exhaust but the HF cats have less than 5000 miles on them so I don’t expect a problem there.

Now I am out of the country until the end of October to work and when I get back there will probably be snow here. So this year has been a total waste with only 2 weeks of driving an unturned car.

 

[Vic]

I had a few people insist that you must remove all the other spark plugs when doing a compression test on one cylinder. One guy even shrilly exclaimed he was sure that you couldn't get any reading at all, unless all the other plugs were removed first. I think he had some kind of emotional issues, I dunno.

There was no technical reason given, which is suspicious in itself, but I think their concept might be that if the other cylinders are sucking air out of the plenum, you might not read a true value for the cylinder with the gauge on it. The plenum is certainly less than atmospheric at idle, but while bumping it over at very low RPM with a button, it still just isn't a freaking Hoover vacuum in there. It just isn't turning fast enough, long enough, to draw down the plenum much below atmospheric with just a few revs. Seemed a little unlikely to me.

So I tried it both ways, and found that it makes almost no difference. Anybody else notice the same thing? Or maybe the opposite?

Maybe if the plenum was very small, as it may be on some cars, then possibly you might notice a difference in compression readings on one cylinder, between all-other-plugs-in, verses all-other-plugs-out. But I think that with the Gen2 plenum, there is so much air volume in there, that after cranking it over for a few revs, or enough to get a good reading, the other cylinders haven't depleted the plenum of air to any noticeable degree. Even with the throttle closed, some air still gets sucked in. Otherwise the car would choke and stall when you release the throttle, right? (Of Course!)

Removing all-other-plugs seems like a good practice, but without much real effect in our case, at least thats what I found. Contrary experience?

Any thoughts on this?

(other than "Thats just the way its done", or "Thats the way we always done it")!!

 

[dave6666]

I've seen this asked before, but never answered. A related question...

Is everyone doing this on a cold engine?

Reason why is, that of course a hot engine would **** to work on, plus you're not supposed to pull the plugs on hot heads, but all the books say that compression tests should be done on an engine that is at operating temperature.

So what is it... A hot or cold test?

 

[ViperTony]

I've seen this asked before, but never answered. A related question...

Is everyone doing this on a cold engine?

Reason why is, that of course a hot engine would **** to work on, plus you're not supposed to pull the plugs on hot heads, but all the books say that compression tests should be done on an engine that is at operating temperature.

So what is it... A hot or cold test?

My test was neither hot or cold but warm.

 

[Tim]

I didn’t burn myself but the engine was still very warm. All the plugs were removed. I used a battery tender to keep the cranking speed up and the throttle was wide open. Subtract 3% per 1000 ft of density altitude change from sea level puts me with in spec of a factory cam and this cam is not factory nor are the heads. I would say that this appears good.
The vacuum is low at 8” and I can’t find a leak but does not mean there isn’t. Or is my cam degreed incorrectly or there could be something else?

 

[dave6666]

So no concerns about spark plugs threads on warm to very warm heads?

Hell, I guess all I have to do is wait until August, park the car in the sun for about an hour with the hood up, and we're at about 140 F then! I knew these Texas summers were good for something.